1. Field of Endeavor
This invention relates to chemical polymer compositions, methods of synthesis, and fabrication methods for devices regarding polymers capable of displaying shape memory behavior and which can first be polymerized to a linear or branched polymeric structure, having thermoplastic properties, subsequently processed into a device through processes typical of polymer melts, solutions, and dispersions and then crosslinked to a shape memory thermoset polymer retaining the processed shape.
2. State of Technology
Shape memory polymers (SMPs) are useful for a diverse set of engineering applications. Because SMPs can retain fixed secondary shapes and recover their original shapes upon heating, their applications are often directed at, but are not limited to, the biomedical industry. For example, an SMP-based suture anchor for graft fixation called Morphix® received FDA approval in February 2009 and has recently been implanted into humans for the first time. An SMP-based interventional microactuator device for treating ischemic stroke is currently being subjected to animal testing at the Texas A&M Institute for Preclinical Studies. SMPs have also received attention for applications outside the medical industry. Raytheon© is currently investigating SMP foams for implementation in thermally-activated wing-deployment systems.
While much progress has been made in the development of new shape memory polymers (SMPs) for engineering applications, difficulties in SMP processing have occurred because many chemically crosslinked SMPs are currently produced in a one-step polymerization of monomers and crosslinking agents. Covalently bonded chemically crosslinked SMPs offer numerous advantages over physically crosslinked SMPs, which include superior cyclic recoverable strains, higher rubbery modulus values, and higher toughness values. These thermoset SMPs are traditionally synthesized either by photo-polymerization or heat-curing of liquid monomers. The chemical reactions that occur during polymerization often result in volume change, which makes complex molding difficult. Thermoset polymers cannot be melted down, so traditional thermoplastic processing methods such as injection molding cannot be used to re-shape chemically crosslinked SMPs to fix deformities. Ultimately, current problems in SMP synthesis have limited the mass-production of complex SMP devices. Without the use of injection molding, the mass-production of complex SMP-based products is neither economically feasible nor advantageous.
What is needed, therefore, is a material that can be melt-processed as a thermoplastic and then crosslinked during a secondary step to fix its final shape. This idea of inducing chemical crosslinking into thermoplastic polymer chains is not in itself novel: it dates back to the 19th century, when the process of vulcanization was developed by Charles Goodyear.16 Late 20th Century projects such as those of Le Roy (Le Roy, et al, Societe Nationale des Poudres et Explosifs (Paris, FR), United States, 1982) and Goyert (Goyert et al, Bayer Aktiengesellschaft, Levertusen Del., United States, 1988) achieved successful crosslinking of thermoplastic polyurethanes and acrylates using irradiation, and Bezuidenhout, et al. U.S. Pat. No. 7,538,163 in 2009 for the development of other chemical mechanisms of post-polymerization urethane crosslinking.
Other have, more recently, investigated post-polymerization crosslinking in thermoplastic polyacrylate systems. However, none of these works have specifically aimed to apply the concept of post-polymerization crosslinking to the synthesis, characterization, and optimization of the thermo-mechanical properties of shape memory polymers with transition temperatures in the range relevant for biomedical applications by tailoring the chemistries of the polymer systems to maximize susceptibility for post-polymerization crosslinking. Furthermore, to our knowledge no prior work had the objective to place crosslinking sites predominantly uniformly spaced along the polymer chain to provide very sharp actuation transitions.